1. Field of the Invention
The present invention relates to zoom lenses suitable for use as image-taking lenses included in image pickup apparatuses such as video cameras, monitoring cameras, digital still cameras, broadcast cameras, and silver-halide film cameras.
2. Description of the Related Art
Image-taking optical systems included in image pickup apparatuses, such as video cameras, monitoring cameras, and digital still cameras, employing solid-state image pickup devices are desired to be zoom lenses having wide angles of view, high zoom ratios, and high optical performance. Such a demand is satisfied by some known zoom lenses each including four lens units: first to fourth lens units having positive, negative, positive, and positive refractive powers, respectively, in order from an object side to an image side.
Some of such zoom lenses are of a rear-focusing type in which the second lens unit is moved for zooming and the fourth lens unit is responsible for focusing and correction of image-plane variation accompanying zooming, as disclosed in U.S. Pat. Nos. 7,400,453 and 6,084,722.
Zoom lenses included in recent image pickup apparatuses, such as video cameras and monitoring cameras, are desired to be small, to have wide angles of view and high zoom ratios, and to have high optical performance over the entire zoom range.
To reduce the overall size of such a zoom lens, the refractive powers of the lens units included in the zoom lens can be increased. However, if the refractive powers of the lens units are simply increased, aberration variation accompanying zooming is increased, making it difficult to obtain good optical performance over the entire zoom range.
Compared with a case of a zoom lens in which the first lens unit is moved for focusing, the first lens unit of a rear-focusing zoom lens has a small effective diameter, and it is therefore easy to reduce the overall size of the zoom lens. Instead, aberration variation accompanying focusing is increased, and it is therefore difficult to obtain high optical performance over the entire object-distance range from an object at infinity to a near object.
Considering the facts described above, to reduce the overall size and increase the angle of view of such a rear-focusing zoom lens including four lens units while maintaining a certain level of zoom ratio, it is important to appropriately design the second and third lens units, specifically, the refractive powers of the second and third lens units, the numbers of negative lens elements included in the second and third lens units, the shapes and arrangement of the negative lens elements, and so forth.